Propose a new way to continue a trace

Author: cleptric

README.md

@@ -7,3 +7,4 @@ For creating a new RFC see [workflow](text/0001-workflow.md).
 
 * [0001-workflow](text/0001-workflow.md): the workflow RFC
 * [0004-import-reorg](text/0004-import-reorg.md): Sentry import reorganization
+* [0014-continue-traces](text/00XX-continue-traces.md): Continue trace on `start_transaction`

text/0014-continue-traces.md

@@ -0,0 +1,63 @@
+* Start Date: 2022-09-26
+* RFC Type: feature
+* RFC PR: https://github.com/getsentry/rfcs/pull/14
+
+# Summary
+
+This RFC proposes a new way to continue a trace when creating nested transactions.
+
+# Motivation
+
+The current way we propagate `sentry-trace` and `baggage`, is to pass a correctly populated `TransactionContext` as the first argument to `startTransaction()`.
+
+```php
+use Sentry\Tracing\TransactionContext;
+use function Sentry\startTransaction;
+
+$transactionContext = TransactionContext::continueFromHeaders($sentryTraceHeader, $baggageHeader);
+$transaction = startTransaction($transactionContext);
+
+```
+
+In case someone starts another nested transaction without passing in any context, a new trace will be started and the Dynamic Sampling Context is lost as well.
+
+# Options Considered
+
+## Add TransactionContext::fromParent()
+
+```php
+use Sentry\Tracing\TransactionContext;
+use function Sentry\startTransaction;
+
+$transactionContext = TransactionContext::fromParent($transaction);
+$transaction = startTransaction($transactionContext);
+
+public static function fromParent(Transaction $transaction)
+{
+    $context = new self();
+    $context->traceId = $transaction->getTraceId();
+    $context->parentSpanId = $transaction->getParentSpanId();
+    $context->sampled = $transaction->getSampled();
+    $context->getMetadata()->setBaggage($transaction->getBaggage());
+
+    return $context;
+}
+```
+
+## Add a third argument to `startTransaction()`
+
+```php
+use Sentry\Tracing\TransactionContext;
+use function Sentry\startTransaction;
+
+$transactionContext = new TransactionContext();
+$transaction = startTransaction($transactionContext, [], bool $continueTrace = true);
+```
+
+This would require making the SDKs aware of the current request.
+In PHP, we could rely on `$_SERVER['HTTP_SENTRY_TRACE]` and `$_SERVER['HTTP_BAGGAGE]`, but this is not possible in all languages.
+
+# Unresolved questions
+
+* Can we rely on `SentrySdk::getCurrentHub()->getTransaction()` to fetch the current transaction to be passed into `TransactionContext::fromParent()` ?
+* How would we make `TransactionContext::__construct()` aware of the current request?